1. Field of the Invention
The present invention relates to a process for producing a polybutadiene having a high cis-1,4-structure content with reduced gel formation, by catalytically polymerizing 1,3-butadiene.
2. Description of the Related Art
It is well known that various types of catalysts have been provided for the polymerization of 1,3-butadiene. Particularly, since a high cis-1,4-polybutadiene, namely a polybutadiene having a high content of a cis-1,4-structure, has excellent thermal and mechanical properties, a variety of polymerization catalysts for the production of high cis-1,4-polybutadiene have been developed.
For example, Canadian Patent No. 795,860 discloses a process for producing a high cis-1,4-polybutadiene by polymerizing 1,3-butadiene in the presence of a catalyst prepared by fully mixing a hydrocarbylaluminum compound with water, aging the mixture, and mixing the aging product with cobalt dioctoate, in a medium containing at least 20% of benzene.
Japanese Examined Patent Publication No. 38-1243 discloses a process for the production of a high cis-1,4-polybutadiene by polymerizing 1,3-butadiene in the presence of a catalyst comprising a cobalt compound, an acid metal halide, an alkylaluminum compound and water.
U.S. Pat. No. 4,182,814 for P. Bernemann et al. discloses a process for polymerizing 1,3-butadiene in the presence of a catalyst including diethylaluminum chloride, water and cobalt octoate in a polymerization medium consisting of a straight chain or branched chain aliphatic hydrocarbon.
U.S. Pat. No. 4,224,426 for J. Odor discloses a process for producing a high cis-1,4-polybutadiene by polymerizing 1,3-butadiene in the presence of a catalyst comprising a cobalt compound, at least one organic aluminum compound and water. In this process, a polymerization medium comprising a C.sub.5 to C.sub.8 cycloalkane is mixed with an alkyl-substituted benzene in an amount of 0.1 to 2% based on the weight of the cycloalkane. This type of polymerization medium is contributory to causing the process control of the polymerization to be easy and the resultant polymerization product to be a substantially linear high cis-1,4-polybutadiene and to contain substantially no gel.
U.S. Pat. No. 5,397,851 for T. Knauf et al. discloses a process for polymerizing 1,3-butadiene in the presence of a catalyst consisting of divalent cobalt salt, alkylaluminum chloride and two different types of trialkylaluminums in a polymerization medium consisting of a mixture of an organic solvent free from aromatic compounds with water.
Polym. Commun., Vol. 32, 514 1991! discloses a process for polymerizing 1,3-butadiene in the presence of a catalyst comprising cobalt acetylacetonate and methyl alumoxane.
However, the above-mentioned conventional catalysts for the polymerization of 1,3-butadiene are disadvantageous in that in the case where the polymerization medium contains no aromatic solvent, the resultant polymer contains a fraction having double bonds and thus gel formation easily occurs, or the catalysts exhibit a low catalytic activity. Accordingly, there has been a strong demand for an improved catalyst.
The high cis-1,4-polybutadiene having a low degree of branching in the polymer chain (namely a high degree of linearity of the polymer chain) is excellent in wear resistance, resistance to exothermic phenomenon and resilient elasticity. However, when the polymerization is carried out in a polymerization medium system containing no aromatic compound, the resultant high cis-1,4-polybutadiene has a higher degree of branching of polymer chain than that produced in a polymerization medium system containing an aromatic compound. Accordingly, there has been a strong demand for a process capable of producing a high cis-1,4-polybutadiene having a low degree of branching of polymer chain even in a polymerization medium system containing no aromatic compound.